It has been shown that many, perhaps most, types of malignant cells are recognized as foreign by our immune surveillance mechanisms. It has also been shown that a series of programmed changes in cell surface antigens occurs during normal embryological development and that such changes can be monitored by immunochemical means. Interestingly, many antigens normally found on specific cell types in embryos are also found on malignant cells. The exact significance of these observations is not yet understood. We believe that research on both cancerous growth and normal development would be greatly facilitated by new immunochemical reagents proposed here. Under a small grant funded by Caltech's President's Fund we have 1) designed an "ideal" immunochemical reagent based on our best knowledge of biocompatible materials and immunochemistry; 2) synthesized uniform polymeric spheres approximately 1000 Angstrom units in diameter; 3) covalently bonded antibodies and fluorescent dyes to the spheres; and 4) demonstrated the feasibility of their use as tagged markers for studies of cell surface antigens. We now propose to extend our preliminary studies. Several types of immunochemical reagents are proposed. Some will carry radioactive tags for use in radioimmune assays and radioautography. Some will also carry fluorescent tags which will serve as visual markers for light microscopy. Finally, they will also serve as readily visible markers for the study of cell surfaces in the scanning electron microscope. We plan to explore the effectiveness of these new reagents in studies of the nature, location and distribution of the interesting antigenic sites known to be present on the surface of immune related murine tumor cell lines. In a parallel study we propose to modify the reagents in such a way as to prepare what might be called immunochemical "smart bombs" which are designed to attach specifically to cultured tumor cell lines and destroy them without seriously affecting other cell types.